Exchange

Question 1

How does an organism’s size relate to their surface area to volume ratio?

Answer 1

The larger the organism, the lower the surface area to volume ratio.

Question 2

How does an organism’s surface area to volume ratio relate to their metabolic rate?

Answer 2

The smaller the surface area to volume ratio, the higher the metabolic rate.

Question 3

How might a large organism adapt to compensate for its small surface area to volume ratio?

Answer 3

Changes that increase surface area e.g. folding; body parts become larger e.g. elephant’s ears; elongating shape; developing a specialised gas exchange surface.

Question 4

Why do multicellular organisms require specialised gas exchange surfaces?

Answer 4

Their smaller surface area to volume ratio means the distance that needs to be crossed is larger and substances cannot easily enter the cells as in a single-celled organism.

Question 5

Name three features of an efficient gas exchange surface.

Answer 5

1. Large surface area, e.g. folded membranes in mitochondria.
2. Thin/short distance, e.g. wall of capillaries.
3. Steep concentration gradient, maintained by blood supply or ventilation, e.g. alveoli.

Question 6

Why can’t insects use their bodies as an exchange surface?

Answer 6

They have a waterproof chitin exoskeleton and a small surface area to volume ratio in order to conserve water.

Question 7

Name and describe the three main features of an insect’s gas transport system.

Answer 7

● Spiracles= holes on the body’s surface which may be opened or closed by a valve for gas or water exchange.

● Tracheae= large tubes extending through all body tissues, supported by rings to prevent collapse.

● Tracheoles= smaller branches dividing off the tracheae.

Question 8

Explain the process of gas exchange in insects.

Answer 8

● Gases move in and out of the tracheae through the spiracles.

● A diffusion gradient allows oxygen to diffuse into
the body tissue while waste CO2 diffuses out.

● Contraction of muscles in the tracheae allows
mass movement of air in and out.

Question 9

Why can’t fish use their bodies as an exchange surface?

Answer 9

They have a waterproof, impermeable outer membrane and a small surface area to volume ratio.

Question 10

Name and describe the two main features of a fish’s gas transport system.

Answer 10

Gills= located within the body, supported by arches, along which are multiple projections of gill filaments, which are stacked up in piles.

Lamellae= at right angles to the gill filaments, give an increased surface area. Blood and water flow across them in opposite directions (countercurrent exchange system).

Question 11

Explain the process of gas exchange in fish.

Answer 11

● The fish opens its mouth to enable water to flow in, then closes its mouth to increase pressure.

● The water passes over the lamellae, and the oxygen diffuses into the bloodstream.

● Waste carbon dioxide diffuses into the water and flows back out of the gills.

Question 12

How does the countercurrent exchange system maximise oxygen absorbed by the fish?

Answer 12

Maintains a steep concentration gradient, as water is always next to blood of a lower oxygen concentration. Keeps rate of diffusion constant and enables 80% of available oxygen to be absorbed.

Question 13

Name and describe three adaptations of a leaf that allow efficient gas exchange.

Answer 13

1. Thin and flat to provide short diffusion pathway and large surface area to volume ratio.
2. Many minute pores in the underside of the leaf (stomata) allow gases to easily enter.
3. Air spaces in the mesophyll allow gases to move around the leaf, facilitating photosynthesis.

Question 14

How do plants limit their water loss while still allowing gases to be exchanged?

Answer 14

Stomata regulated by guard cells which allows them to open and close as needed. Most stay closed to prevent water loss while some open to let oxygen in.

Question 15

Describe the pathway taken by air as it enters the mammalian gaseous exchange system.

Answer 15

Nasal cavity → trachea → bronchi → bronchioles → alveoli

Question 16

Describe the function of the nasal cavity in the mammalian gaseous exchange system.

Answer 16

A good blood supply warms and moistens the air entering the lungs. Goblet cells in the membrane secrete mucus which traps dust and bacteria.

Question 17

Describe the trachea and its function in the mammalian gaseous exchange system.

Answer 17

● Wide tube supported by C-shaped cartilage to keep the air passage open during pressure changes.

● Lined by ciliated epithelium cells which move mucus towards the throat to be swallowed, preventing lung infections.

● Carries air to the bronchi.

Question 18

Describe the bronchi and their function in the mammalian gaseous exchange system.

Answer 18

● Like the trachea they are supported by rings of cartilage and are lined by ciliated epithelium cells.

● However, they are narrower and there are two of them, one for each lung.

● Allow passage of air into the bronchioles.

Question 19

Describe the bronchioles and their function in the mammalian gaseous exchange system.

Answer 19

● Narrower than the bronchi.

● Do not need to be kept open by cartilage, therefore mostly have only muscle and elastic fibres so that they can contract and relax easily during ventilation.

● Allow passage of air into the alveoli.

Question 20

Describe the alveoli and their function in the mammalian gaseous exchange system.

Answer 20

● Mini air sacs, lined with epithelium cells, site of gas exchange.

● Walls only one cell thick, covered with a network of capillaries, 300 million in each lung, all of which facilitates gas diffusion.

Question 21

Explain the process of inspiration and the changes that occur throughout the thorax.

Answer 21

● External intercostal muscles contract (while internal relax), pulling the ribs up and out.

● Diaphragm contracts and flattens.

● Volume of the thorax increases.

● Air pressure outside the lungs is therefore higher than the air pressure inside, so air moves in to rebalance.

Question 22

Explain the process of expiration and the changes that occur throughout the thorax.

Answer 22

● External intercostal muscles relax (while internal contract), bringing the ribs down and in.

● Diaphragm relaxes and domes upwards.

● Volume of the thorax decreases.

● Air pressure inside the lungs is therefore higher than the air pressure outside, so air moves out to rebalance.

Question 23

What is tidal volume?

Answer 23

The volume of air we breathe in and out during each breath at rest.

Question 24

What is breathing rate?

Answer 24

The number of breaths we take per minute.

Question 25

How do you calculate pulmonary ventilation rate?

Answer 25

Tidal volume x breathing rate. These can be measured using a spirometer, a device which records volume changes onto a graph as a person breathes.

Question 26

Define digestion.

Answer 26

The hydrolysis of large, insoluble molecules into smaller molecules that can be absorbed across cell membranes.

Question 27

Which enzymes are involved in carbohydrate digestion? Where are they found?

Answer 27

● Amylase in mouth

● Maltase, sucrase, lactase in membrane of small intestine

Question 28

What are the substrates and products of the carbohydrate digestive enzymes?

Answer 28

● Amylase → starch into smaller polysaccharides

● Maltase → maltose into 2 x glucose

● Sucrase → sucrose into glucose and fructose

● Lactase → lactose into glucose and galactose

Question 29

Where are lipids digested?

Answer 29

The small intestine.

Question 30

What needs to happen before lipids can be digested?

Answer 30

They must be emulsified by bile salts produced by the liver. This breaks down large fat molecules into smaller, soluble molecules called micelles, increasing surface area.

Question 31

How are lipids digested?

Answer 31

Lipase hydrolyses the ester bond between the monoglycerides and fatty acids.

Question 32

Which enzymes are involved in protein digestion? What is their role?

Answer 32

● Endopeptidases= break between specific amino acids in the middle of a polypeptide.

● Exopeptidases= break between specific amino acids at the end of a polypeptide.

● Dipeptidases= break dipeptides into amino acids.

Question 33

How are certain molecules absorbed into the ileum despite a negative concentration gradient?

Answer 33

Through co-transport.

Question 34

Which molecules require co-transport?

Answer 34

Amino acids and monosaccharides.

Question 35

Explain how sodium ions are involved in co-transport.

Answer 35

Sodium ions (Na+) are actively transported out of the cell into the lumen, creating a diffusion gradient. Nutrients are then taken up into the cells along with Na+ ions.

Question 36

Why do fatty acids and monoglycerides not require co-transport?

Answer 36

The molecules are nonpolar, meaning they can easily diffuse across the membrane of the epithelial cells.

Question 37

Describe the structure of haemoglobin.

Answer 37

Globular, water soluble. Consists of four polypeptide chains, each carrying a haem group (quaternary structure).

Question 38

Describe the role of haemoglobin.

Answer 38

Present in red blood cells. Oxygen molecules bind to the haem groups and are carried around the body to where they are needed in respiring tissues.

Question 39

Name three factors affecting oxygen-haemoglobin binding.

Answer 39

1. Partial pressure/concentration of oxygen.
2. Partial pressure/concentration of carbon dioxide.
3. Saturation of haemoglobin with oxygen.

Question 40

How does partial pressure of oxygen affect oxygen-haemoglobin binding?

Answer 40

As partial pressure of oxygen increases, the affinity of haemoglobin for oxygen also increases, so oxygen binds tightly to haemoglobin. When partial pressure is low, oxygen is released from haemoglobin.

Question 41

How does partial pressure of carbon dioxide affect oxygen-haemoglobin binding?

Answer 41

As partial pressure of carbon dioxide increases, the conditions become acidic causing haemoglobin to change shape. The affinity of haemoglobin for oxygen therefore decreases, so oxygen is released from haemoglobin. This is known as the Bohr effect.

Question 42

How does saturation of haemoglobin with oxygen affect oxygen-haemoglobin binding?

Answer 42

It is hard for the first oxygen molecule to bind. Once it does, it changes the shape to make it easier for the second and third molecules to bind, known as positive cooperativity. It is then slightly harder for the fourth oxygen molecule to bind because there is a low chance of finding a binding site.

Question 43

Explain why oxygen binds to haemoglobin in the lungs.

Answer 43

● Partial pressure of oxygen is high.

● Low concentration of carbon dioxide in the lungs, so affinity is high.

● Positive cooperativity (after the first oxygen molecule binds, binding of subsequent molecules is easier).

Question 44

Explain why oxygen is released from haemoglobin in respiring tissues.

Answer 44

● Partial pressure of oxygen is low

● High concentration of carbon dioxide in respiring tissues, so affinity decreases.

Question 45

What do oxyhaemoglobin dissociation curves show?

Answer 45

Saturation of haemoglobin with oxygen (in %), plotted against partial pressure of oxygen (in kPa). Curves further to the left show the haemoglobin has a higher affinity for oxygen.

Question 46

How does carbon dioxide affect the position of an oxyhaemoglobin dissociation curve?

Answer 46

Curve shifts to the right because haemoglobin’s affinity for oxygen has decreased.

Question 47

Name three common features of a mammalian circulatory system.

Answer 47

1. Suitable medium for transport, water-based to allow substances to dissolve.
2. Means of moving the medium and maintaining pressure throughout the body, such as the heart.
3. Means of controlling flow so it remains unidirectional, such as valves.

Question 48

Relate the structure of the chambers to their function.

Answer 48

● Atria: thin-walled and elastic, so they can stretch when filled with blood

● Ventricles: thick muscular walls pump blood under high pressure. The left ventricle is thicker than the right because it has to pump blood all the way around the body.

Question 49

Relate the structure of the vessels to their function.

Answer 49

● Arteries have thick walls to handle high pressure without tearing, and are muscular and elastic to control blood flow.

● Veins have thin walls due to lower pressure, therefore requiring valves to ensure blood doesn’t flow backwards. Have less muscular and elastic tissue as they don’t have to control blood flow.

Question 50

Why are two pumps (left and right) needed instead of one?

Answer 50

To maintain blood pressure around the whole body. When blood passes through the narrow capillaries of the lungs, the pressure drops sharply and therefore would not be flowing strongly enough to continue around the whole body. Therefore it is returned to the heart to increase the pressure.

Question 51

Describe what happens during cardiac diastole.

Answer 51

The heart is relaxed. Blood enters the atria, increasing the pressure and pushing open the atrioventricular valves. This allows blood to flow into the ventricles. Pressure in the heart is lower than in the arteries, so semilunar valves remain closed.

Question 52

Describe what happens during atrial systole.

Answer 52

The atria contract, pushing any remaining blood into the ventricles.

Question 53

Describe what happens during ventricular systole.

Answer 53

The ventricles contract. The pressure increases, closing the atrioventricular valves to prevent backflow, and opening the semilunar valves. Blood flows into the arteries.

Question 54

Name the nodes involved in heart contraction and where they are situated.

Answer 54

● Sinoatrial node (SAN)= wall of right atrium.

● Atrioventricular node (AVN)= in
between the two atria.

Question 55

What does myogenic mean?

Answer 55

The heart’s contraction is initiated from within the muscle itself, rather than by nerve impulses.

Question 56

Explain how the heart contracts.

Answer 56

● SAN initiates and spreads impulse across the atria, so they contract.

● AVN receives, delays, and then conveys the impulse down the bundle of His.

● Impulse travels into the Purkinje fibres which branch across the ventricles, so they contract from the bottom up.

Question 57

Why does the impulse need to be delayed?

Answer 57

If the impulse spread straight from the atria into the ventricles, there would not be enough time for all the blood to pass through and for the valves to close.

Question 58

How is the structure of capillaries suited to their function?

Answer 58

● Walls are only one cell thick; short diffusion pathway.

● Very narrow, so can permeate tissues and red blood cells can lie flat against the wall, effectively delivering oxygen to tissues.

● Numerous and highly branched, providing a large surface area.

Question 59

What is tissue fluid?

Answer 59

A watery substance containing glucose, amino acids, oxygen, and other nutrients. It supplies these to the cells, while also removing any waste materials.

Question 60

How is tissue fluid formed?

Answer 60

As blood is pumped through increasingly small vessels, this creates hydrostatic pressure which forces fluid out of the capillaries. It bathes the cells, and then returns to the capillaries when the hydrostatic pressure is low enough.

Question 61

How is water transported in plants?

Answer 61

Through xylem vessels; long, continuous columns that also provide structural support to the stem.

Question 62

Explain the cohesion-tension theory.

Answer 62

Water molecules form hydrogen bonds with each other, causing them to ‘stick’ together (cohesion). The surface tension of the water also creates this sticking effect. Therefore as water is lost through transpiration, more can be drawn up the stem.

Question 63

What are the three components of phloem vessels?

Answer 63

● Sieve tube elements= form a tube to transport sucrose in the dissolved form of sap.

● Companion cells= involved in ATP production for active loading of sucrose into sieve tubes.

● Plasmodesmata= gaps between cell walls where the cytoplasm links, allowing substances to flow.

Question 64

Name the process whereby organic materials are transported around the plant.

Answer 64

Translocation

Question 65

How does sucrose in the leaf move into the phloem?

Answer 65

Sucrose enters companion cells of the phloem vessels by active loading, which uses ATP and a diffusion gradient of hydrogen ions. Sucrose then diffuses from companion cells into the sieve tube elements through the plasmodesmata.

Question 66

How do phloem vessels transport sucrose around the plant?

Answer 66

As sucrose moves into the tube elements, water potential inside the phloem is reduced. This causes water to enter via osmosis from the xylem and increases hydrostatic pressure. Water moves along the sieve tube towards areas of lower hydrostatic pressure. Sucrose diffuses into surrounding cells where it is needed.

Question 67

Give evidence for the mass flow hypothesis of translocation.

Answer 67

● Sap is released when a stem is cut, therefore there must be pressure in the phloem.

● There is a higher sucrose concentration in the leaves than the roots.

● Increasing sucrose levels in the leaves results in increased sucroses in the phloem.

Question 68

Give evidence against the mass flow hypothesis of translocation.

Answer 68

● The structure of sieve tubes seems to hinder mass flow.

● Not all solutes move at the same speed, as they would in mass flow.

● Sucrose is delivered at the same rate throughout the plant, rather than to areas with the lowest sucrose concentration first.

Question 69

How can ringing experiments be used to investigate transport in plants?

Answer 69

The bark and phloem of a tree are removed in a ring, leaving behind the xylem. Eventually the tissues above the missing ring swells due to accumulation of sucrose as the tissue below begins to die. Therefore sucrose must be transported in the phloem.

Question 70

How can tracing experiments be used to investigate transport in plants?

Answer 70

Plants are grown in the presence of radioactive CO2, which will be incorporated into the plant’s sugars. Using autoradiography, we can see that the areas exposed to radiation correspond to where the phloem is.